Recently, ceramics composed principally of silicon nitride (Si.sub.3 N.sub.4) have found significant use as ceramic components for machines or as vessel coating. This material is known to have many good characteristics such as high oxidation resistance at high temperatures (1400.degree. C.), good mechanican strength at high--1400 temperatures, and good hardness at high temperature.
Strength of this material is related to density and it has been found that the densification property of silicon nitride, sintered under atmospheric pressure, is very inferior. Therefore, it has been considered important to employ high pressure when a product of good strength is desired. This is routinely referred to as hot pressing of silicon nitride. However, in spite of the use of hot pressing, the bend strength of simple Si.sub.3 N.sub.4 has not been as high as desired at high temperatures. Accordingly, other avenues of strength improvement have been sought such as through the use of additives which operate as a low temperature liquid phase to facilitate densification and not significantly imparing the creep resistance of the ceramic body at high temperatures. These added materials have included relatively large amounts of chromium oxide, zinc oxide, nickel oxide, titanium oxide, cerium oxide, magnesium oxide, yttrium oxide and others, ranging in excess of 20% (wt.) of the matrix material. Silicon nitride with these particular additives tends to form a structure having a strength level which does not usually exceed 50 KSI at high temperatures. In one instance (U.S. Pat. No. 3,830,652 to Gaza) did the prior art obtain strength levels in excess of 50 KSI. In this instance, the concern was for physical characteristics useful for turbine elements; hardness, oxidation resistance (inertness) and transverse rupture strength. Gaza explored metal oxide additives to a Si.sub.3 N.sub.4 system which ranged in amounts related solely to machine element usage. The additions were added in amounts up to 20%.
However, commercial cutting tools today exhibit the same or better physical properties that were the focus of Gaza's work. For example, commercial Al.sub.2 O.sub.3 or TiC tools have excellent hardness at high temperatures and have high resistance to oxidation and have transverse rupture strengths at high temperatures which range up to 100,000 psi. Strength is considered the most important feature because of the necessity to withstand forces imposed on the tool material by the tool fixture and by the resistance of the stock material, particularly at heavy depths of cutting. These forces become unusually exaggerated when cutting ferrous material such as cast iron at high speeds and feeds. Without increased strength, it is believed by those skilled in the art that further improvements in tool life cannot be achieved. Since the strength level of Si.sub.3 N.sub.4 is equal to or lower than commercial materials now available, it has been rejected as a tool material candidate with little hope in improving tool life.
In only one known instance has the art attempted to employ Si.sub.3 N.sub.4 directly as a cutting tool material and this was for use only on hypereutectic aluminum alloys. This attempt is set forth in a Japanese Pat. No. 49-113803 (10-30-1974) by Kazutaka Ohgo, appearing in Chemical Abstracts, Volume 84, 1976, page 286 (84:21440t). In this work, silicon nitride was sintered (as opposed to hot pressing) and metal oxide spinels were employed in solid solution in the silicon nitride matrix. The spinels were formed by a mixture of divalent and trivalent metal oxides (including magnesium oxide and Y.sub.2 O.sub.3). Only a quarternary system was employed involving Si.sub.3 N.sub.4, SiO.sub.2, MgO, and Y.sub.2 O.sub.3. This produced many secondary phases which weakened the physical characteristics, particularly strength, thermal conductivity, and increased the thermal coefficient of expansion. A loss of these physical characteristics make it most difficult to obtain even equivalent performance to commercially available tools when applied to a rigorous cutting environment such as interrupted cutting on cast iron. The cutting operation was of very short duration (2 minutes) of continuous machining and at low metal removal rates (cutting speeds of 1000 sfm, 0.012 inches per rev. of feed and 0.060 inches of depth of cut and metal removal of 8.64 in..sup.3 /min.). This type of test information, of course, did not investigate cutting applications where large forces are applied to the tool, did not investigate the elimination of spinel additives, did not investigate heavy cutting against rough surfaces such as cast iron, nor continuous cutting for periods of several hours or greater, nor did it explore intermittent, interrupted high speed cutting at speeds of 4000-5000 sfm at heavy feeds and depths of cutting. The demonstrated wear of 0.006-0.008 inches, in Ohgo's work, for 2 minutes of cutting time is highly excessive when compared to the goals of the present invention. Therefore, this work did not demonstrate that Si.sub.3 N.sub.4 possessed sufficient characteristics to be used as a tool material on ferrous materials which apply large bend forces to the tool.
Moreover, the art has been possessed of sufficient knowledge in the making of Si.sub.3 N.sub.4 with additives for many years; during this long term no effort was made to apply this material as a cutting tool against cast iron. This tends to support the contention of this invention that if tool life is dramatically increased for certain Si.sub.3 N.sub.4 composites when used for machining cast iron, there must be some unobvious characteristics independent of strength that layed undiscovered to promote this new use.
This invention has discovered a correlation between a thermal shock parameter and promotion of prolonged life in Si.sub.3 N.sub.4 materials when used as a cutting tool on cast iron. This parameter consists of (KS/.alpha.E) where K is thermal conductivity of the material, S is the modulus of rupture, .alpha. the coefficient of thermal expansion, and E is Young's modulus. E can be eliminated from the parameter since it remains substantially constant for the contemplated variation in ceramic chemistry which controls this parameter. This parameter must exceed 26 lbs/in..sup.2 as minimal if significant improvement in tool life is to be obtained. It has been further discovered that a simple ternary ceramic system (Si.sub.3 N.sub.4.SiO.sub.2. low temp. liquid phase) with SiO.sub.2 present, not as an additive, but as an inherent reaction product of heating Si.sub.3 N.sub.4, serves as the proper mechanism for achieving the required thermal shock parameter. The low temperature liquid phase must be one which produces a small amount of a highly refractory silicate which will reside totally in the grain boundary of the matrix.
There are many other physical characteristics beyond its thermal shock parameter that should be improved in silicon nitride if it is to be successful as a tool material for cutting cast iron. As indicated earlier, the densification of the material has been a point of concern and has been alleviated by use of hot pressing techniques and oxide additives. This has permitted the density to be elevated close to theoretical density, but improving density by itself through increasing amounts of oxide leads to a decrease in several other physical properties. Investigators have failed to perceive this interplay of physical characteristics.
More importantly, known silicon nitride compositions, when used as a cutting tool against relatively rough surfaces such as cast iron, exhibit a failure mode under such circumstances is typically due to thermal shock as opposed to the more desirable mode by wear. Further the attainable hardness level and general rigidity of the known silicon nitride composites have yet to be comparable to commercial cutting tools.